Circular chart recorders have been used for many years on gas wells to determine the flow rate and especially the quantity of gas flow to the gas collection system. An accurate record is desired because this is the means for determining how much money the well owner is paid for the gas from the well. When a well is new, the gas flow may be nearly continuous from the well into the collection pipe system, but as the well gets older, the ratio decreases markedly of on:off time of the valve regulating flow from the gas well.
The charts might be 24-hour charts, or 7- or 30-day charts and, typically, two marking pens mark lines on this circular chart, one showing static pressure in the pipe collection system and the second showing differential pressure across an orifice in the flow line to the pipe collection system. These two signals are multiplied together and then the square root is taken of the product in order to get flow rate, and this is detailed in the AGA Report No. 3, well known in the gas well industry. The charts are taken off the chart recorder, usually placed at the well head, and taken to a chart reader who usually utilizes a planimeter or other similar means to determine the area under each curve. This has been standard practice for many years, and the accuracy with which the charts are manually read determines the accuracy of payment to the well owner.
A number of different chart conditions contribute to the difficulty of accuracy of reading. For example, some charts show a solid, wide line where the pen has oscillated back and forth rapidly relative to the slow rotation of the chart, and the chart reader has to make a guess as to where the pen was located the majority of the time. Another type of chart difficult to read is a sunburst chart, where there are wide swings in the pen, and with the wide oscillations of the pen close together relative to the rotational speed of the chart. Such a chart, again, is very difficult to read accurately.
A prior art system which attempted to solve this was shown in U.S. Pat. No. 3,322,339, wherein the mechanical system was physically cionnected to the chart recorder and mechanically acted as a planimeter to calculate the area under the curves, thus not requiring a manual operator to trace over the curve on the chart. This mechanical system placed an extra load on the movement of the pen, which could cause inaccuracies due to the extra loading. Also, it had many moving parts, and lost motion at the pivot points could cause further inaccuracies.
U.S. Pat. No. 3,742,515 disclosed an analyzer for a chart recorder which utilized electrical signals to produce a triangular wave traced along the edge of the recording chart, with a frequency of this triangular wave being proportional to the amplitude of the analog signal. However, this did not obtain an electrical signal showing the total quantity of fluid flow, nor even rate of fluid flow.
U.S. Pat. Nos. 1,877,810 and 2,611,812 show gas flow recording devices which utilize variable inductances or movable core transformers responsive to the fluid pressure, and then another such inductance device, usually at a remote location for a remote recorder, to reconvert the electrical signal to a mechanical movement so as to move a recording pen on a rotatable chart. These patents disclosed a remote reading instrument, but still left unsolved the problem of how to accurately read the chart and determine the gas flow rate therefrom.
These prior art devices were subject to mechanical wear, lost motion, and temperature effects, an also included the difficulties of errors in manual chart integration. Further, they were subject to errors due to clock drive of the disc chart and pen malfunctions, and required a relatively rapidly rotating chart, e.g., a 24-hour chart instead of a 7- or 30-day chart, in order to help minimize errors in determining the quantity of fluid flow.